The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
As an aluminum heat exchanger for use in a car-air conditioning refrigeration cycle, the so-called multi-flow type or parallel-flow type heat exchanger is well-known, wherein the heat exchanger is provided with a plurality of flat tubes disposed in the thickness direction with a fin interposed therebetween and a pair of hollow headers with the ends thereof connected to the flat tubes in a fluid communication. In such heat exchangers, the fin and/or header is made of an aluminum brazing sheet with clad brazing material, and the entire components of the heat exchanger including the fins and headers are integrally brazed in a furnace in a provisionally assembled manner.
In such aluminum heat exchangers, as technique for improving the corrosion resistance, technique for forming a sacrificial corrosion layer on a heat exchanger tube is generally used.
For example, in the technique disclosed in Japanese Unexamined Laid-open Publication No. H4-15496 (Patent Document 1), Zn is thermally sprayed onto the surface of the heat exchanger tube to form a sacrificial corrosive layer by diffusing the Zn in the surface portion of the tube.
Other than the aforementioned Zn thermal spraying treatment, as disclosed by Japanese Unexamined Laid-open Patent Publication No. H11-131254 (Patent Document 2) and Japanese Patent No. 3,437,023 (Patent Document 3), to improve corrosion resistance, technique for forming an anti-corrosion film by subjecting a surface of an aluminum product such as a heat exchanger to chemical conversion treatment is also employed.
As disclosed in the aforementioned Patent Document 1, in the technique for thermally spraying Zn on the surface of a heat exchanger tube, if the Zn adhering amount is increased at the time of thermally spraying Zn on the surface of the heat exchanging tube, excessive Zn will be diffused in the brazing portion (i.e., fillet), causing preferential corrosion of the fillet. This in turn causes the so-called fin detachment in which a fin is detached from a tube. Accordingly, it is preferable to decrease the Zn adhering amount. However, since the Zn thermal spraying for spraying a low amount of Zn becomes unstable, adhesion amount becomes uneven, resulting in unstable sacrificial corrosion layer throughout the entire sprayed area. As a result, partial corrosion occurs when used for a long time period, causing pit corrosion.
Furthermore, as shown in the aforementioned Patent Documents 2 and 3, in the technology which forms a corrosion resistant coat by chemical conversion treatment on a heat exchanger surface, although a uniform corrosion resistant coat can be formed broadly, the corrosion resistance could not be maintained for a long period of time, and therefore generation of pit corrosion, etc., can not be prevented effectively.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.
Other objects and advantages of the present invention will be apparent from the following preferred embodiments.